As is known, many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may be defined by a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material. More specifically, the web of packaging material is unwound off a reel and fed through an aseptic chamber on the packaging machine, where it is sterilized, e.g. by applying a sterilizing agent such as hydrogen peroxide, which is subsequently evaporated by heating, and/or by subjecting the packaging material to radiation of appropriate wavelength and intensity; and the sterilized web is maintained in a closed, sterile environment, and is folded into a cylinder and sealed longitudinally to form a continuous tube in known manner.
The tube of packaging material, which in effect forms an extension of the aseptic chamber, is fed in a vertical direction, is filled with the sterilized or sterile-processed food product, and is fed through a sealing device to form the individual packages. More specifically, in the sealing device, the tube is sealed at a number of equally spaced cross sections to form pillow packs connected to one another by transverse sealing strips, i.e. extending perpendicularly to the travelling direction of the tube. The pillow packs are separated from one another by cutting the relative transverse sealing strips, and are conveyed to a folding station where they are folded mechanically to form respective finished parallelepiped-shaped packages.
Packaging machines are known, as described for example in European Patent EP-B-0887265, which comprise two chain conveyors defining respective endless paths and fitted with respective numbers of sealing jaws. The two paths have respective branches substantially facing and parallel to each other, and between which the tube of packaging material is fed so that the jaws on one conveyor cooperate with corresponding jaws on the other conveyor along said branches of the respective paths, to grip the tube at a number of successive cross sections, and to seal and cut the packs.
Packaging machines are also known comprising only two pairs of jaws, which act alternately on the tube of packaging material to grip and seal, e.g. heat seal, it along a number of equally spaced cross sections.
As the sealing operation is completed, a cutter, carried for example by one of the jaws in each pair, is activated, and interacts with the tube of packaging material to cut it along a centreline of the cross section just sealed, and so detach a pillow pack from the bottom end of the tube of packaging material. The bottom end being sealed transversely, the relative jaws, on reaching the bottom dead-centre position, can be opened to avoid interfering with the top portion of the tube. At the same time, the other pair of jaws, operated in exactly the same way, moves down from the top dead-centre position, and repeats the above grip/form, seal and cut process.
On both types of packaging machines, the tube portion gripped between each pair of jaws is typically sealed by heating means carried by one of the jaws, and which locally melt the layers of heat-seal plastic material gripped tightly between the jaws.
To reduce the time taken to melt the packaging material locally, and so increase package output, heating means comprising ultrasonic sealing devices are widely used.
As described, for example, in EP-B-615907, such devices substantially comprise a mechanical-vibration generator or sonotrode, and an anvil, which are fitted to respective jaws in each pair, and cooperate with each other to heat the packaging material by ultrasonic vibration.
More specifically, a sonotrode is a sealing tool which is vibrated by one or more disks of piezoelectric material. The disks are supplied with alternating voltage, and generate mechanical vibration, the energy of which is related to the effective voltage or electric current supply value.
More specifically, for correct, complete sealing, the sonotrode must be supplied with a particularly high voltage, e.g. of about a thousand volts.
Being movable with the relative jaws, the sonotrodes must be powered electrically by an electric energy source fixed in a given position along the path of the tube of packaging material.
In other words, electric energy must be transferred from a fixed source to a moving user device along a portion of the path traveled by the user device.
This can be done in known manner by equipping the jaws supporting the sonotrodes with brushes, preferably made of carbon, which, along a predetermined portion of the path traveled by the brushes, slide along respective copper power supply bars fixed to the packaging machine frame.
Rapid wear of the brushes and unstable contact between the brushes and the power supply bars are the major drawbacks of this method, and which tend to get worse as the output rate of the packaging machine increases.
To eliminate these drawbacks, it has been proposed, as described for example in EP-04105565, to employ a method based on transferring electric energy from the fixed source to the moving user device by electromagnetic induction.
Electric energy is transferred from a transmitting unit, fixed to the packaging machine frame, to a receiving unit fitted to one of the jaws in a relative pair.
The transmitting unit has a fixed magnetic core fitted with a primary winding connected to the electric energy source, while the receiving unit has a movable magnetic core integral with the body of the relative jaw and fitted with a secondary winding connected to the relative sonotrode.
More specifically, the electric energy source is a voltage generator, which supplies the primary winding to generate in it an electric current and associated magnetic field which, as the receiving unit travels past the transmitting unit, induces electric current in the secondary winding to supply the sonotrode.
There being no contact between the receiving and transmitting units, the above method solves the wear problem.
On the other hand, known induction electric energy transfer systems are unsatisfactory in that, for correct, complete sealing, the sonotrode would have to be supplied with particularly high voltage, which would have to be transferred by induction from the primary to the secondary winding.
Since, in operating conditions, the environment between the primary and secondary windings is damp and contaminated with conducting substances, such transfer would entail a high risk of electric discharges on the primary and secondary windings, thus possibly resulting in damage to and impaired operation of the packaging machine.
The Applicant has found this to be particularly so in the case of the primary winding, which has extensive exposed surfaces, to reduce the effects of nonhomogeneous electric current distribution (known as “skin effect”) and is therefore difficult to insulate.
Induction transfer systems are also unsatisfactory on account of the electric energy supplied by the voltage source being supplied to the primary winding largely in the form of voltage and to a lesser degree in the form of electric current, whereas the electric current induced in the secondary winding and, therefore, the voltage supplying the sonotrode are proportional the electric current flow in the primary winding.